Document FR 2 776 540 describes such a process of forming a barrier layer, and in particular documents FR 2 783 667, FR 2 792 854, and FR 2 847 912 describe various examples of devices enabling such a deposit to be made.
The person skilled in the art knows that in cold plasma methods, and in particular in the method known in the art as plasma enhanced chemical vapor deposition (PECVD), both the accuracy of the instantaneous microwave energy level that is emitted, and the waveform of the power emitted during the treatment cycle, constitute some of the main factors that enable coating deposition to present quality that is substantially constant, in other words that make it possible over time to obtain containers that are of substantially identical quality. A fortiori, in industrial installations for large capacity production having a multiplicity of deposition devices, it is important to control accurately the instantaneous microwave energy level delivered to all of the cavities in all of the devices of the installation in order to minimize differences in performance between devices within a single machine, or indeed between different machines, and thus differences in quality between containers processed respectively in a plurality of devices.
Various pieces of equipment are indeed known that are available for accurately adjusting such microwave energy levels (circulators, devices for measuring the real microwave power emitted, tuning stubs, . . . ). Nevertheless, such pieces of equipment are expensive, and thus difficult to envisage in an industrial installation where low cost price is of permanent concern; furthermore, such pieces of equipment are bulky and therefore difficult to install in an industrial machine, particularly when of the rotary type, which is already very cluttered with equipment and in which little space remains available; finally, effective and efficient implementation of equipment of that kind requires precise calibration adjustments that can be performed only by qualified personnel, not always available in industrial installations for mass production in which there is a constant concern for the technical means used to be simple to implement and operate.
Thus, in order to satisfy the requirement for reducing dispersion in the characteristics of the coating deposited on containers in a high-speed industrial process, it is necessary to find a specific and inexpensive solution for accurately controlling the operation of the magnetron.
It is reminded that a magnetron, which lies at the core of any system using microwaves, serves to transform an input high voltage (of several kilovolts (kV)) into an electromagnetic wave at a given ultra-high (microwave) frequency. The high voltage is delivered by a high voltage supply that is suitable for transforming a low voltage power supply (in particular at the voltage of a conventional electrical power supply network, e.g. 400 volts (V) three-phase) into a high voltage that is modulated as a function of the microwave energy desired at the output from the magnetron. For each model of magnetron, magnetron manufacturers provide basic curves serving to define the characteristics of the high voltage supply. Thus, for each model of magnetron, it is possible to obtain in particular a curve plotting variation in anode current as a function of emitted microwave power, a curve plotting variation in electrical efficiency as a function of the emitted microwave power, and a curve plotting variation in the high voltage to be applied to the magnetron as a function of the microwave power emitted.
The electrical efficiency of a magnetron is substantially stable for a given emitted microwave power and it varies little as a function of emitted microwave power (in a typical example of a magnetron, variation in electrical efficiency is of the order of 2.8% for emitted microwave power varying over the range 350 watts (W) to 900 W).
Nevertheless, all of those magnetron characteristics are valid only when the magnetron is coupled to a load that is said to be “matched”, i.e. a load that does not reflect back towards the magnetron a fraction of the microwave energy that it receives therefrom.
Unfortunately, with devices of the kind to which the invention is more specifically intended, i.e. devices that are used for depositing a coating on a container of thermoplastic material with the help of a low pressure plasma by exciting a precursor gas with UHF electromagnetic waves in an evacuated cavity of cylindrical shape receiving said container, not only is the load coupled to the magnetron not matched, but in addition it does not remain constant over time, and it varies very quickly (over a period of the order of a few milliseconds). These variations in load are inherent to the conditions under which the plasma is formed in the cavity for a given emitted mean microwave power (operating conditions for the device as set by the operator as an operating setpoint):                at the beginning of the process, the plasma is not yet established; the load coupled to the magnetron is poorly matched and it reflects a large amount of energy;        thereafter the plasma becomes established within the cavity; the load coupled to the magnetron is matched better and it reflects less energy.        
It is emphasized at this point that the mean power setpoint does not change between those two operating stages. The variations in the voltage and the current applied to the magnetron are associated solely with the behavior of the magnetron faced with varying amounts of reflected energy.
In an attempt to maintain the microwave power actually emitted by the magnetron at the setpoint value, it is known to implement anode current regulation: a proportionality coefficient is predetermined between anode current and emitted microwave power (where this characteristic can form part of the data provided by the manufacturer of the magnetron); in operation, the value of the anode current is measured continuously and a proportional correction is applied to the anode current as a function of variations in the load on the high voltage generator so as to maintain the microwave power emitted by the magnetron as constant as possible relative to the setpoint power.
The speed of power supply regulation is selected to be relatively slow (response time greater than 100 milliseconds (ms)), while the changeover from the strongly mismatched load condition to the better-matched load condition is very short and can correspond to one period of the high voltage (e.g. of the order of 10 ms to 20 ms). As a result, mainly during the start-up stage, the above-mentioned unbalance can extend over a plurality of high voltage pulses, with a large amount of unbalance in the power delivered by the high voltage power supply for the emitted microwave power being substantially analogous.
For a more concrete idea, FIG. 1 of the accompanying drawings is a graph showing the operation of a typical example of a magnetron and plotting as a function of time (along the abscissa, expressed in seconds), variation in the high voltage applied to the terminals of the magnetron (continuous line curve, plotted up the ordinate on the right-hand scale expressed in volts), and corresponding variation in regulated anode current under the above-mentioned conditions (dashed line curve plotted up the ordinate on the left-hand scale, expressed in milliamps).
It can be seen that for the group constituted by the first two cycles (to the left in the graph), the high voltage presents a lowest value of −3.6 kilovolts (kV); the percentage of energy reflected by the poorly matched load (the plasma is not yet established) is high. For the group constituted by the following cycles, the high voltage takes the value of −4 kV; the plasma is established, and the load is better matched, with a smaller percentage of energy being reflected.
The anode current applied to the generator is regulated relatively slowly, with a response time of the order of 40 ms. The instantaneous peak powers of the pulses PA (belonging to the group of first cycles) and of the pulses PB (belonging to the group of following cycles) are as follows:                pulse PA: the anode current has a value of 360 milliamps (mA); the manufacturer of the magnetron gives a proportionality coefficient of 3 w of microwaves per milliamp, so the instantaneous microwave power delivered by the magnetron is 360×3, giving 1080 w; and        pulse PB: the anode current has a value of 305 mA; the instantaneous microwave power delivered by the magnetron is 305×3, i.e. 915 W.        
For the two pulses PA and PB shown in FIG. 1 as being the closest together during the changeover in operating conditions, it can be considered that, given the relative slowness with which the power supply is regulated, the internal parameters of power supply operation remain unchanged. The difference between the microwave power delivered by the magnetron, due to variation in the matching of the load coupled to the magnetron which continues to deliver mean microwave power that is substantially analogous in both circumstances, is about 15%, and is therefore very large.
As a result, the operating conditions of present devices fitted with high voltage power supplies using anode current regulation for the purpose of maintaining the microwave power emitted by the magnetron at a setpoint value are not optimized because the high voltage power supply is subjected to large and rapid variations in power.